Explanation of 2s-2p mixing effect on molecular orbital energy levels for homonuclear diatomic molecules/molecular ions : When mathematically mixing atomic orbitals of two atoms to form molecular orbitals for homonuclear diatomic molecules/molecular ions, one might assume that the atoms' atomic s-orbitals only mix with each other and atomic p-orbitals only mix with each other, because we assume that the atomic s- and p- orbitals are far enough from each other energetically that there is no mixing of the two. But if we are mixing atomic orbitals of atoms with ≤ 3 electrons in their p orbitals (as in the case with Li through N in period 2), there is a chance for some 2s-2p mixing: the 2s of one atom mixes somewhat with the 2p of the other atom. This results in an increase in the energy level of the resulting σ 2p molecular orbital, which results in the two π 2p molecular orbitals being lower in energy than the σ

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Unformatted text preview: 2p orbital in the cases of B 2 , C 2 , and N 2 (as well as Li 2 and Be 2 considerations). If, however, there are > 3 electrons in the p orbitals of the atoms (as in the case of O through Ne of period 2), then the greater electronic repulsions that result from these greater electronic densities in the p orbitals results in there being no significant 2s-2p orbital mixing. This results in the expected energetic order of molecular orbitals where the σ 2p is lower in energy than the two π 2p orbitals, in the cases of O 2 , F 2 , and Ne 2 . So, just keep in mind that when filling molecular orbital electronic configurations for period 2 elements, be sure to fill the σ 2p orbital before the two π 2p orbitals for O 2 , F 2 , and Ne 2 , and be sure to fill the two π 2p orbitals before the σ 2p orbital for B 2 , C 2 , and N 2 (as well as Li 2 and Be 2 considerations)....
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